The invention relates to a method and a circuit arrangement for the ignition of an internal-combustion engine, as defined in the preambles to the independent claims.
Known ignition systems, which are commonly used in motor vehicles, comprise an ignition coil, an ignition distributor and spark plugs. A high voltage is generated in the ignition coil, then supplied via the ignition distributor to the individual spark plugs. The generated high voltage is typically in a range between 20 kV and 30 kV. The high voltage leads to a voltage arc-over at the spark-plug electrodes, which ignites the fuel/air mixture. It is also known to provide an electronic ignition distribution instead of a rotating, so-called distributor finger in an ignition distributor. Because of the high energy that must be available for the ignition process, the ignition coil must have a large volume, and, from the output of the ignition coil, the circuit arrangement must be high-voltage-proof for avoiding voltage arc-overs onto the vehicle chassis.
U.S. Pat. No. 5,113,839 discloses an ignition method in which a high-frequency ignition is triggered by the application of an AC voltage to the spark-plug electrodes. A supply unit for 200 V is required for executing the method, and the efficiency is reduced.
DE-A1 196 25 422 discloses a hybrid ignition circuit for an internal-combustion engine, in which the ignition-spark breakdown is generated by a capacitor discharge, and the combustion of the engine spark is maintained by an AC voltage in a flow converter.
EP 0 482 127 B1 discloses a method in which the voltage for the voltage arc-over and the combustion phase is generated via a resonant transformation. The problem here is that the method is energy-consuming, and the spark plug is subjected to a great deal of wear.
It is the object of the invention to provide a method and a circuit arrangement with which an ignition system of the above-described type is improved so as to have a particularly compact, energy-saving design, with the spark plugs being subjected to little wear.
The object is accomplished by the features of the independent claims. Modifications and advantageous embodiments ensue from the dependent claims and the description.
The method in accordance with the invention involves supplying electrical energy for triggering a voltage arc-over at the spark-plug electrodes with a self-induction method, in which a transformer supplies a voltage to the spark plug and an abrupt reduction in the current flow through the primary side of the transformer causes the voltage arc-over, and the combustion phase is maintained by means of a resonant voltage transformation, with a resonant frequency being determined by inductive and capacitive elements on the secondary side of a transformer.
In a preferred embodiment of the invention, the same transformer is used for the self-induction and for maintaining the combustion phase.
It is advantageous to supply the spark plug with a high-frequency AC voltage during the combustion phase. The spark plug is preferably supplied with electrical power of a frequency higher than 100 kHz during the combustion phase. In a further preferred embodiment of the method, the spark plug is supplied with electrical power of a frequency in a range of 1 MHz to a few hundred MHz in the combustion phase.
In a further preferred embodiment, during the combustion phase, the spark plug is supplied with a high-frequency AC voltage, which is superposed with a DC voltage or a low-frequency AC voltage.
The circuit arrangement of the invention for the ignition of an internal-combustion engine is characterized by the fact that supply elements for the spark plug have at least four switches in a bridge circuit for supplying a voltage to the electrodes.
A further circuit arrangement of the invention for the ignition of an internal-combustion engine is characterized by the fact that supply elements for the spark plug have at least two switches in a push-pull circuit with a transformer for supplying a voltage to the electrodes.
In a further preferred embodiment, two switches and two voltage sources are disposed in a partial-bridge circuit.
At least one coil and/or one transformer is or are preferably disposed, as an inductance of a resonant circuit, between the switches and electrodes. The transformer represents an inductance, at least in regions, and is a component of both the resonant circuit and the voltage transformation of the self-induction circuit.
The bridge circuit is preferably connected to an actuation unit that opens and closes the switches according to the requirements of the ignition phase and the combustion phase.
In a preferred embodiment, the resonant transformation circuit comprises at least one Collins filter.
In an especially preferred embodiment, the resonant transformation circuit comprises at least one series resonant circuit.
A common transformer is advantageously provided for generating a self-induction voltage for the voltage arc-over and for generating the resonant voltage transformation.
It is beneficial to provide a separate resonant transformation circuit for each spark plug. The advantage is that transformation circuit and the spark plug can be integrated into a unit.
Elements are advantageously provided for combining the self-induction, the resonant transformation and the spark plug into an ignition-unit component.
The provision of elements for combining the self-induction, resonant transformation, spark plug and actuation electronics into an ignition-unit component is especially beneficial.
In an advantageous embodiment, an ignition unit can be connected to elements for self-induction and resonant transformation, and the spark plug, by means of a plug connection.
In an advantageous modification of the arrangement according to the invention, the resonant transformation circuit is connected at its input to an AC-voltage source and a DC-voltage source.
It is particularly advantageous to integrate the entire arrangement into a cylinder head.